Some grammatical rules and errrorrs in writing essays:

In summaries, keep to one tense. If you were to switch, it was confusing.

Subjects and verbs should agree; when it don't, they are in error.

Sentence fragments are. Being incomplete.

Comma splices connect independent clauses, they do so with a comma, however, this is inadequate adhesive for such independent elements. Use a semicolon; use a period.

A run-on sentence blurs thoughts together like a comma splice without the comma it does not separate independent clauses.